Analysis and Countermeasures for Transverse Cracks Beneath the Overlay Layer on Reactor Core Makeup Water Tank Cylinders

Literature Overview

This study addresses a critical quality issue encountered during the fabrication of nuclear island reactor core makeup water tank cylinders, specifically the occurrence of transverse cracks beneath the overlay layer. The reactor core makeup water tank is a safety-related pressure vessel that supplies borated water to the reactor core during loss-of-coolant accidents, making it subject to the highest quality standards in nuclear construction. The overlay layer, typically composed of stainless steel or nickel-based alloy, is applied to provide corrosion resistance against borated water and to meet the material requirements specified in nuclear codes. The investigation of the transverse cracks beneath the overlay layer is of paramount importance because such cracks can compromise the structural integrity of the vessel and the reliability of the nuclear safety system.

Core Technical Findings

Crack Characteristics

The transverse cracks identified beneath the overlay layer exhibit the following characteristics:

The transverse orientation of the cracks is significant because it indicates that the cracking is driven by transverse residual stresses, which are typically higher than longitudinal residual stresses in circumferential welds on cylindrical shells.

Root Cause Analysis

The investigation identified multiple contributing factors to the transverse cracking:

1. High Residual Stresses: The overlay welding process introduces significant residual stresses, particularly in the transverse direction, due to the constrained cooling of the overlay weld bead on the thick cylinder shell. The transverse residual stress can reach values of 300-400 MPa, exceeding the yield strength of the low-alloy steel base metal in the heat-affected zone.
2. Hydrogen Embrittlement: The base metal in the HAZ, particularly the high-hardness martensitic zone, is susceptible to hydrogen-induced cracking. Hydrogen from the welding process diffuses into the HAZ and accumulates at stress concentration sites, promoting crack initiation and propagation.
3. Microstructural Hardness: The HAZ of the low-alloy steel base metal, particularly in the fine-grained zone adjacent to the overlay, develops hardness values of 350-450 HV due to martensitic transformation. This high hardness zone is highly susceptible to hydrogen-induced cracking.
4. Thermal Cycling Effects: The multi-pass overlay welding process subjects the base metal to repeated thermal cycling, which can promote the formation of brittle microstructures and increase the susceptibility to cracking.

Metallurgical Evidence

Metallographic examination of the crack sites reveals:

• The crack initiates at the interface between the overlay and the base metal HAZ, propagating through the HAZ into the base metal.
• The fracture surface exhibits a mixed intergranular and transgranular morphology, consistent with hydrogen-induced cracking.
• Hydrogen microembrittlement is observed in the HAZ ahead of the crack tip, characterized by microvoids and microcracks along prior austenite grain boundaries.
• The hardness of the HAZ at the crack site is 380-420 HV, significantly higher than the base metal hardness of 200-240 HV.

Process Analysis and Key Technical Points

Welding Procedure Qualification

The original welding procedure qualification did not adequately address the cracking susceptibility of the base metal HAZ. The following improvements are recommended:

The increased preheat temperature reduces the cooling rate in the HAZ, limiting the formation of hard martensite and reducing the susceptibility to hydrogen-induced cracking. The post-weld heat treatment relieves residual stresses and allows for the tempering of any martensitic microstructure in the HAZ, further reducing the cracking susceptibility.

Residual Stress Management

Residual stress measurement using the hole-drilling method and X-ray diffraction confirms that the transverse residual stresses in the overlay weld region reach 300-400 MPa before post-weld heat treatment. After PWHT at 600°C for 2 hours, the residual stresses are reduced to below 100 MPa, which is well below the threshold for hydrogen-induced cracking. The following residual stress management strategies are recommended:

1. Preheat and Interpass Heating: Maintaining the base metal temperature above 150°C during welding reduces the thermal gradient and the resulting residual stresses.
2. Post-Weld Heat Treatment: A full PWHT at 590-620°C is essential for relieving residual stresses and tempering the HAZ microstructure.
3. Stress Relief Welds: In cases where PWHT is not feasible, stress relief welds can be applied along the overlay weld to provide a path for stress relaxation.
4. Vibration Stress Relief: Post-weld vibration stress relief can be used as a supplementary method to reduce residual stresses, particularly in the transverse direction.

Hydrogen Control

Hydrogen control is critical for preventing hydrogen-induced cracking in the overlay welds on low-alloy steel base metals. The following measures are recommended:

• Use of low-hydrogen filler metals and electrodes, with hydrogen content below 5 ml/100g.
• Thorough drying of welding consumables according to the manufacturer's specifications.
• Use of dry, uncontaminated shielding gas with moisture content below 0.1%.
• Application of post-weld baking at 200-250°C for 1-2 hours to allow hydrogen diffusion out of the weld and HAZ.
• Avoidance of rapid cooling of the weld by using thermal blankets or controlled cooling rates.

Engineering Practice Integration

Nuclear Quality Requirements

The reactor core makeup water tank is a nuclear safety-related component that must meet the stringent quality requirements of the applicable nuclear codes, including:

• ASME III: Division 1, Section III, for the design and fabrication of nuclear pressure vessels.
• ASME IX: For the qualification of welding procedures and welders.
• ASME V: For the non-destructive examination requirements.
• 10 CFR 50 and 52: For the regulatory requirements governing nuclear facility construction and operation.

The overlay welds must be qualified in accordance with ASME IX, and the welding procedure must demonstrate the ability to produce welds free of cracks, lack of fusion, and other unacceptable defects. The qualification records must be maintained for the lifetime of the component and are subject to regulatory inspection.

Inspection and Acceptance Criteria

The following inspection requirements are specified for overlay welds on nuclear safety-related components:

The acceptance criteria for overlay welds on nuclear components are more stringent than for conventional pressure vessels, reflecting the safety-critical nature of the application.

Lessons Learned and Corrective Actions

The investigation of the transverse cracks led to the following corrective actions:

1. Revision of the welding procedure to include higher preheat temperatures and PWHT.
2. Implementation of a hydrogen control program, including consumable drying and post-weld baking.
3. Introduction of residual stress measurement and monitoring during the welding process.
4. Enhanced welder training on the importance of procedure adherence and the consequences of deviations.
5. Establishment of a root cause analysis protocol for any weld defects identified during fabrication.

Key Questions and Reflections

The investigation of the transverse cracks beneath the overlay layer on the reactor core makeup water tank cylinder raises several important questions. First, the interaction between residual stresses, hydrogen, and microstructural hardness in the HAZ is complex, and the relative contribution of each factor to the cracking susceptibility requires further quantitative analysis. Second, the effectiveness of PWHT in preventing cracking must be validated for each specific welding procedure and base metal condition, as the response to PWHT can vary depending on the alloy composition and the welding parameters. Third, the long-term performance of the overlay welds under nuclear service conditions, including irradiation, thermal cycling, and corrosion, must be evaluated through accelerated testing and aging studies.

Study Insights and Implications

This investigation highlights the critical importance of understanding the interaction between residual stresses, hydrogen, and microstructural factors in the prevention of cracking in overlay welds on nuclear safety-related components. The transverse cracks beneath the overlay layer are a manifestation of the complex metallurgical and mechanical challenges associated with overlay welding on thick-section low-alloy steel cylinders. The corrective actions implemented, including increased preheat, PWHT, and hydrogen control, demonstrate that the cracking can be effectively prevented through a systematic approach to welding procedure optimization and quality control. Engineers working on nuclear pressure vessel fabrication must adopt a rigorous, evidence-based approach to welding procedure qualification and in-process monitoring to ensure the integrity and reliability of safety-related components.